An assembled battery has been used as a battery for an electric vehicle (EV) and a hybrid electric vehicle (HEV). Such an assembled battery is constructed with a number of secondary cells connected in series to produce a high voltage of about between 100 and 400 volts. For example, an assemble battery of 300 volts is constructed with 150 lead cells (2 volts per cell) connected in series, 250 nickel hydride cells (1.2 volts per cell) connected in series, or 80 lithium ion cells (about 3.6 volts per cell) connected in series.
A secondary cell, in particular, a lithium ion cell is sensitive to overcharging and overdischarging. If the secondary cell is used outside its rated voltage range, capacity of the secondary cell significantly decreases, and the secondary cell becomes overheated. Therefore, a cell voltage of the secondary cell is monitored by a cell voltage monitor circuit. The cell voltage monitor circuit detects that the cell voltage exceeds the rated voltage range.
JP-A-2003-70171 discloses a measurement circuit that measures a cell voltage of each of lithium ion cells that construct an assembled battery. The measurement circuit includes differential amplifier circuits constructed with operational amplifiers. The differential amplifier circuits are provided to the lithium ion cells, respectively. Each differential amplifier circuit uses a virtual ground as a reference potential to measure the cell voltage. The virtual ground has a potential equal to a potential at the midpoint of the assemble battery.
An integrated circuit (IC) 1 shown in FIG. 15 includes a cell voltage monitor circuit 2 that monitors cell voltages VV1-VV4 of secondary cells BC1-BC4, which are connected in series to construct an assembled battery 3. The IC 1 is formed by using junction isolation techniques. The cell voltage monitor circuit 2 uses a potential at a negative terminal of the cell BC1 as a reference potential (i.e., ground potential GND) to measure the cell voltages VV1-VV4. The cell BC1 is arranged on the lowest potential side of the assembled battery 3. The cell voltage monitor circuit 2 monitors the cell voltages VV1-VV4 by comparing the cell voltages VV1-VV4 with a reference voltage Vr. The reference voltage Vr is generated by a reference voltage generation circuit 8.
The cell voltage monitor circuit 2 includes sub monitor circuits 2a-2d that monitor the cell voltages VV1-VV4 of the cells BC1-BC4, respectively. As an example, the sub monitor circuits 2b includes an operational amplifier 4, a differential amplifier circuit 6, and a comparator 7. The operational amplifier circuit 4 is configured as a voltage follower. The differential amplifier circuit 6 includes resistors R1-R4 and an operational amplifier 5. The differential amplifier circuit 6 outputs the cell voltage VV2, which is a difference between a voltage V2 at a positive terminal of the cell BC2 and a voltage V1 at a negative terminal of the cell BC2. The comparator 7 compares the cell voltage VV2 with the reference voltage Vr. Although each of the sub monitor circuits 2a, 2c and 2d are configured in a similar manner to the sub monitor circuit 2b, the sub monitor circuit 2a has no differential amplifier circuit 6. A monitor signal generation circuit 9 outputs a monitor signal Sd in accordance with output signals of the sub monitor circuits 2a-2d. 
The cell voltage monitor circuit 2 receives the terminal voltages V1-V4 of the cells BC1-BC4 with respect to the ground potential GND and generates the cell voltages VV2-VV4 by using the differential amplifier circuit 6. Therefore, the cell voltage monitor circuit 2 needs the operational amplifier circuits 4, 5 in addition to the comparator 7. As a result, the IC 1 requires a large chip size and a large current consumption.